Code review comment for ~canonical-bootstack/charm-openstack-service-checks:check-loadbalancers

Hi,
Thanks for the review and feedback.
I've fixed all, and refactored the code to make it more flexible/reasonable.

Could you please review again when you get the time?

NOTE: for image check, it hardcoded `amp_img_tag` to `octavia-amphora` at the moment, and it's disabled by default. Work in progress to find a proper/minimal way to set it.